The invention pertains to physical vapor deposition target assemblies and methods of forming physical vapor deposition target assemblies. In particular applications, the invention pertains to physical vapor deposition target assemblies comprising a physical vapor deposition target joined to a support comprising carbon fibers dispersed in a metal matrix.
Physical vapor deposition targets have wide application in fabrication processes where thin films are desired, and include, for example, sputtering targets. An exemplary application for physical vapor deposition processes, such as, for example, sputtering processes, is in semiconductor processing applications for forming thin films across semiconductor substrates.
A physical vapor deposition target can comprise any of numerous metallic elements and alloys, or can comprise ceramic materials. In operation, a physical vapor deposition target is exposed to ions or atoms which impact a surface of the target and are utilized to eject material from the physical vapor deposition target surface toward a substrate. The ejected material lands on the substrate to form a thin film over the substrate. The ejected material is typically displaced from the sputtering surface in the form of small, discrete pieces comprising a few atoms or less of target material. The pieces are generally desired to be uniform in size and composition relative to one another. However, problems can occur in which some the ejected material is in the form of xe2x80x9cparticlesxe2x80x9d or xe2x80x9csplatsxe2x80x9d. The terms xe2x80x9cparticlexe2x80x9d and xe2x80x9csplatxe2x80x9d refer to chunks of ejected material that are much larger than the average size of the pieces ejected from the sputtering surface. The particles can adversely affect properties of a film deposited from a target, and accordingly it is generally desired to reduce particle generation. Particle generation can be particularly severe when there is a large thermal stress in the target arising from large differences of thermal expansion coefficient between the target and the backing plate, and from high temperature due to high power deposition.
Physical vapor deposition targets are retained in a chamber or other apparatus during a deposition process, and problems can occur in fabricating the targets for such retention. One method of retaining a physical vapor deposition target within an apparatus is to first mount the target to a so-called backing plate. The backing plate is configured to connect the target to the apparatus, and preferably comprises an electrical conductivity which is equal to or greater than the material of the target so that the backing plate does not impede electrical or magnetic flow from the apparatus through the target. A common material utilized for backing plate constructions is copper. The backing plate can be mounted to a target by, for example, bonding the backing plate and target together with solder.
The backing plate will generally comprise a different material than the target, and accordingly will have different physical properties. Among the physical properties which can differ from a backing plate to a target is the coefficient of thermal expansion. If a target has a significantly different coefficient of thermal expansion than a backing plate associated with the target, there can be significant strain introduced at a bond formed between the backing plate and target. Such strain can fatigue the bond and eventually result in separation of the target from the backing plate. Among the targets which can be particularly problematic are targets comprising tungsten, such as targets which consist essentially of, or consist of tungsten; as well as targets which comprise a significant amount of tungsten (i.e., greater than 50 atom % tungsten), such as targets comprising, consisting essentially of, or consisting of tungsten and titanium. The coefficient of thermal expansion for tungsten is 4.5xc3x9710xe2x88x926Kxe2x88x921, whereas the coefficient of thermal expansion for copper is 16.5xc3x9710xe2x88x926Kxe2x88x921. Accordingly, targets comprising a substantial amount of tungsten have significantly different coefficients of thermal expansion than backing plates comprising copper.
Tungsten and tungsten/titanium compositions have applications in semiconductor processing methodologies as, for example, conductive plugs and Al barrier materials. Accordingly, an effort has been made to develop methodologies for physical vapor deposition of tungsten and tungsten/titanium, and specifically an effort has been made to develop methodologies for bonding tungsten-containing targets with copper-containing backing plates. One of the methodologies which has been developed is to utilize a relatively soft solder, such as, for example, a solder comprising indium, to bond the target to the backing plate. The soft solder can then expand and contract to create a flexible bond between the target and backing plate. A difficulty with utilizing indium-containing solders is that the solders have a melting point of about 170xc2x0 C., and can lose structural integrity at temperatures of about 80xc2x0 C. or above. It is common for targets to heat to temperatures of 80xc2x0 C. or above during a sputtering operation, and such can cause the indium-solder bond between the target and backing plate to fail. The failure can cause target separation from the backing plate, and in particularly problematic cases, can cause a target to fall from a backing plate during a physical vapor deposition operation. Also, it can be desired to heat a target to temperatures significantly above 80xc2x0 C. to reduce particle generation during a sputtering operation. Physical vapor deposition processes which are operated only at the relatively cold temperatures at which indium-based solders are stable can be particularly problematic relative to particle generation.
In light of the above-discussed problems, it is desirable to develop new methodologies for retaining physical vapor deposition targets in physical vapor deposition apparatuses, and particularly it is desirable to develop new backing plates and new techniques for mounting physical vapor deposition targets to backing plates.
In one aspect, the invention encompasses a method of forming an assembly of a physical vapor deposition target and support. A physical vapor deposition target is provided. The physical vapor deposition target has a coefficient of thermal expansion of less than 10xc3x9710xe2x88x926Kxe2x88x921. The physical vapor deposition target is joined to a support. The support has a thermal coefficient of expansion of less than 11xc3x9710xe2x88x926Kxe2x88x921.
In another aspect, the invention encompasses an assembly comprising a physical vapor deposition target and a support joined to the physical vapor deposition target. The support comprises carbon fibers and copper.